1. Field of the Invention
This invention relates to a process for manufacturing detergent compositions with predominantly alkoxylated nonionic surfactants having a high powder density and which is free-flowing.
2. Statement of the Related Art
In recent years, there has been an increasing interest in detergents having a comparatively high powder (bulk) density of more than 600 g/l because they require less packaging volume for the same active ingredient content and thus enable a significant reduction of packaging material size. Washing powders of high powder density have been known for some time and include compositions of high soda or silicate content obtained simply by mixing the individual constituents together or by drying aqueous mixtures thereof on shelves or heated rolls, followed by extrusion or spray crystallization. These powders, which have a high specific gravity, tend to cake, generally show inadequate dissolving properties, and cannot be used in modern washing machines with preprogrammed wash cycles. Accordingly, compositions of this type have been replaced by low specific gravity powders having a porous grain structure which are produced by hot spray drying and which, although generally dissolving rapidly, are relatively bulky in terms of packaging and transport.
It is also known that the powder density of spray-dried powders such as these can be increased by subsequently spraying them with liquid or molten nonionic surfactants. By virtue of the favorable washing properties of nonionic surfactants, this also increases the detergency of the powders and avoids the problem of pluming in the exhaust of the spray drying towers which occurs during hot spray drying and which is caused by entrained nonionic material. The process in which the nonionic surfactant is applied to spray-dried polyphosphate only gives powder densities of less than 550 g/l. U.S. Pat. Nos. 3,838,072; 3,849,327; and 3,886,098 describe a similar process in which a granular, porous carrier material is prepared by spray drying a slurry of inorganic salts such as sodium silicate, sodium sulfate and sodium triphosphate; sulfonate surfactants; and soaps; and is subsequently sprayed with a nonionic surfactant in a mixer. In this way, up to 20% by weight of nonionic surfactants may be subsequently applied to the spray-dried carrier material. In order to improve the flow properties, the addition of a powder, for example talcum, finely divided silica or calcined clay, is recommended. A powder-form redeposition inhibitor such as carboxymethyl cellulose may also be subsequently added. The powders charged with nonionic surfactants obtained in this way generally have a powder density of 300 to 600 g/l, with an undesired high of 700 g/l, and a fluidity of up to 76% of that of dry sand. The powder particles vary from 0.075 mm to 3.3 mm and more especially from 0.15 mm to 0.83 mm in size.
Granular detergents having a powder density of 550 to 800 g/l which consist of essentially spherical particles having a certain particle size and which have a fluidity of at least 75% and up to almost 100% based on dry sand, are known from published German Application No. 27 42 683 and U.S. Pat. No. 4,444,673. These known detergents, which are packed in plastic bottles, contain 30 to 80% builders, 2 to 40% surfactants which are mostly nonionic, 0 to 20% other additives, and 0 to 50% fillers, and have a moisture content of 3 to 15%. Although it is disclosed that these detergents may be produced by any method, including spray drying or granulation, the preferred method and only method specifically described is based on a complicated two-stage process in which base beads having a porous outer surface and a more or less absorbent interior are initially prepared by spray drying an aqueous slurry and are then sprayed or impregnated with the liquid or molten nonionic surfactant. Apart from the complicated nature of this process, difficulties are involved in preparing non-tacky granules containing more than 20% of the liquid or low-melting nonionic surfactants. In addition, the products show comparatively unfavorable dissolving properties in cold tap-water, so that undissolved fractions can remain behind in the powder dispenser compartment or in the liquid dispenser container of tumbler-type washing machines of the type commonly used in Europe.
Finally, Canadian Pat. No. 852,173 and corresponding published German patent application No. 17 92 434 describe a process for the production of granular detergents containing 2 to 15% by weight of anionic surfactants, 5 to 20% by weight of nonionic surfactants and 25 to 60% by weight of tripolyphosphate, by spray drying a slurry. The tripolyphosphate used for preparing the slurry must be partly prehydrated. This partial prehydration is critical to the formation of free-flowing powders. This known process yields loose powders having a powder density of less than 550 g/l and, where the nonionic surfactant content is considerably in excess of 15% by weight, only very moderate flow properties. Thus, it is impossible to transfer the powder in defined quantities from a box or bottle into a measuring cup because it does not flow uniformly. On the contrary, when the container is tilted and shaken, however carefully, to dispense the powder, the powder does not flow out uniformly, but instead sticks or shoots uncontrollably out of the opening so that the measuring cup often overflows and relatively large quantities of powder are spilt.
Accordingly, the problem posed was to produce a granular detergent component while avoiding the known disadvantages and which:
(a) has a high powder density so that the packaging volume can be considerably reduced, i.e. to around half that of a conventional spray-dried detergent:
(b) has a much higher content of wash-active substance (about twice normal) so that the detergent develops the same washing power as a conventional spray-dried powder when used in smaller amounts, for example in amounts reduced by half:
(c) is so free-flowing that it may be poured out like a liquid and may be exactly dispensed into a measuring cup simply by tilting the container (despite the resulting high content of nonionic surfactants which are known to increase the tendency of a powder toward caking); and
(d) can be produced by a single-stage process without any particular technical problems arising.
In attempting to solve this problem one is confronted by the following negative aspects:
A spray-drying process carried out under the usual conditions, i.e. by pressure atomization of aqueous suspensions, offers little in the way of a solution to this problem because spray drying generally gives expanded, i.e. porous, granules having correspondingly low powder densities. Although the subsequent addition of or impregnation with liquefied nonionic surfactants would have more or less filled the pores of the granules and increased the powder density accordingly, the two-stage procedure is both time consuming and requires very expensive apparatus because of the need to dispense, mix and granulate large quantities of powder and then to remove the coarser aggregates. In addition, a procedure such as this necessitates the production of relatively strong, i.e. abrasion-resistant, granules. Granules such as these, which normally contain relatively high percentages of sodium silicate as strength promoter, generally show only moderate dissolving properties, particularly in cold water, and frequently have only a limited uptake capacity for liquid or tacky nonionic surfactants.
There are no known processes for directly producing heavy powders of the type in question with a high nonionic surfactant content by spray drying. First, there were serious doubts regarding the spray drying of powders having a high surfactant content, particularly a high nonionic surfactant content, because of the danger of dust explosions and the extensive pluming expected in the exhaust of the spray drying towers. Accordingly, the relevant patents and literature in the art warn against processing highsurfactant mixtures such as these in hot spray drying towers and instead propose incorporating higher percentages of nonionic surfactant in preformed carrier grains by spray granulation. Second, conventional techniques for preparing and further processing the aqueous concentrates (slurries) and subsequent hot spray drying were specifically developed to form porous, loose powders of low powder density. Accordingly, these techniques appeared unsuitable for the production of compact, low-dust powders having approximately double the normal powder density.
There are essentially two known processes for preparing and further processing the slurry. In the semicontinuous process, at least two mixing vessels operating alternatively are used. This inevitably results in prolonged dwell times during which the tripolyphosphate is hydrated and viscosity increases. The spraying of these viscous slurries under pressures of 30 to 70 bar through nozzles which normally have a bore diameter of 2.5 to 4 mm results in the formation of loose powders having a powder density below 400 to 450 g/l. Another process, described in "Soap and Cosmetic Specialities", August 1972, pages 27 to 30, 44 and 46, uses continuous metering, mixing and pumping systems. The individual constituents are continuously weighed or volumetrically measured, premixed and transferred to a homogenizing unit. After passing through a filter, in which relatively coarse agglomerates are removed or broken up, the slurry flows through a second homogenizing unit to a high-pressure pump by which it is pumped to a spray-drying tower under a pressure of 30 to 70 bar. This continuous procedure avoids prolonged dwell times and large increases in the viscosity of the slurry, but also gives powders having a powder density of only 100 to at most 450 g/l. A low content of washing-active substance, which is equivalent to a high content of builder salts, and high spraying pressures promote a higher powder density, although even here the upper limit is at 400 to 450 g/l. To produce more compact heavy powders, therefore, the spray-dried product has to be further processed and mixed with powders of high specific gravity in apparatus specially designed for this purpose. This requires higher plant investment and more work.